Microfluidic device and microtube thereof

ABSTRACT

A micromachined tube (microtube) suitable for microfluidic devices. The microtube is formed by isotropically etching a surface of a first substrate to define therein a channel having an arcuate cross-sectional profile, and forming a substrate structure by bonding the first substrate to a second substrate so that the second substrate overlies and encloses the channel to define a passage having a cross-sectional profile of which at least half is arcuate. The substrate structure is thinned to define the microtube and walls thereof that surround the passage.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of co-pending U.S. patentapplication Ser. No. 12/397,197, filed Mar. 3, 2009, which claims thebenefit of U.S. Provisional Application No. 61/067,882, filed Mar. 3,2008. The contents of these prior applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to micromachining processes anddevices formed thereby. More particularly, this invention relates to aprocess of forming a micromachined tube (microtube) suitable for amicrofluidic device, including but not limited to Coriolis mass flowsensors, density sensors, specific gravity sensors, fuel cellconcentration meters, chemical concentration sensors, temperaturesensors, drug infusion devices, fluid delivery devices, gas deliverydevices, gas sensors, bio sensors, medical sensors, and other devicescapable of making use of a stationary or resonating microtube.

Processes for fabricating resonant mass flow and density sensors usingsilicon micromachining techniques are disclosed in commonly-assignedU.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628. As usedherein, micromachining is a technique for forming very small elements bybulk etching a substrate (e.g., a silicon wafer), and/or by surfacethin-film etching, the latter of which generally involves depositing athin film (e.g., polysilicon or metal) on a sacrificial layer (e.g.,oxide layer) on a substrate surface and then selectively removingportions of the sacrificial layer to free the deposited thin film. Inthe processes disclosed in U.S. Pat. Nos. 6,477,901, 6,647,778,7,351,603 and 7,381,628, wafer bonding and etching techniques are usedto produce a micromachined tube supported above a surface of asubstrate. The tube can be vibrated at resonance, by which the flowrate, density, and/or other properties or parameters of a fluid flowingthrough the tube can be measured.

According to one embodiment of U.S. Pat. No. 6,477,901, a tube is formedusing p-type doped layers and selective etching techniques. The dopedlayers form the walls of the tube, and therefore determine and limit thesize of the tube walls as well as the cross-sectional dimensions of thetube. According to another embodiment of U.S. Pat. No. 6,477,901, a tubeis formed with the use of a silicon-on-insulator (SOI) wafer. The buriedoxide layer of the SOI wafer is used as an etch stop in a manner thatdetermines and can limit the thickness of the tube. In U.S. Pat. No.7,351,603, an epitaxial wafer is employed to avoid the higher cost ofSOI wafers.

The micromachined tubes produced by the processes disclosed in U.S. Pat.Nos. 6,477,901, 6,647,778, 7,351,603 and 7,381,628 have roughlyrectilinear cross-section passages as a result of using an anisotropicdry etching technique, such as reactive ion etching (RIE), dry etching,or deep reactive ion etching (DRIE), or a wet etching technique if thewafer is formed of a (110) oriented silicon. As known in the art,anisotropic etching processes produce a substantially one-directionaletch, yielding the vertical walls of the passages shown within themicrotubes of U.S. Pat. Nos. 6,477,901, 6,647,778, 7,351,603 and7,381,628.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a process for producing a micromachinedtube suitable for microfluidic devices, nonlimiting examples of whichinclude resonating microtubes for mass flow and density sensors,stationary microtubes, diaphragms, and passages for such microfluidicdevices as needles, cannula, pressure sensors, temperature sensors,motion sensors, drug infusion devices, fluid delivery devices, gasdelivery devices, gas sensors, bio sensors, medical sensors, and otherdevices that can employ microtubes.

According to a first aspect of the invention, the process entailsisotropically etching a surface of a first substrate to define therein achannel having an arcuate cross-sectional profile, and forming asubstrate structure by bonding the first substrate to a second substrateso that the second substrate overlies and encloses the channel to definea passage having a cross-sectional profile of which at least half isarcuate. The substrate structure can optionally then be thinned todefine a microtube and walls thereof that surround the passage.

A second aspect of the invention is the various types of microtubesproduced by the process described above.

In view of the above, the present invention provides a process by whichmicrotubes with at least partially arcuate passages can bemicromachined. According to preferred aspects of the invention, achannel can be formed in the second substrate to have an arcuatecross-sectional profile, with the result that the passage has anentirely arcuate cross-sectional profile, nonlimiting examples of whichinclude circular and elliptical cross-sectional shapes. Arcuate passagesproduced by this invention are capable of exhibiting improved dynamicfluid flow as a result of reduced turbulence and stagnant regions withinthese passages, resulting in lower pressure drops and higher flow ratesthrough their microtubes without necessitating an increase in thein-plane (width) and out-of-plane (height) dimensions of the microtubes.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows cross-sectional views of two wafers suitable as startingmaterial for producing a micromachined tube in accordance with anembodiment of this invention.

FIG. 2 shows the wafers of FIG. 1 bonded together to form a wafer stack,and FIG. 3 depicts the wafer stack following etching to form an arcuatechannel in a surface of the wafer stack.

FIG. 4 depicts the result of bonding the wafer stack of FIG. 3 to athird wafer having an arcuate channel formed in its surface, with theresult that an enclosed round passage is defined within the resultingwafer structure by the two arcuate channels.

FIG. 5 depicts the result of thinning the wafer structure of FIG. 4.

FIG. 6 depicts the result of selectively thinning portions of the waferstructure to either side of the round passage.

FIGS. 7 and 8 depict the wafer structure of FIG. 6 bonded to asubstrate.

FIG. 9 depicts the result of removing remaining portions of the waferstructure of FIGS. 7 and 8 to define external walls of a microtubehaving a freestanding portion suspended over the substrate.

FIG. 10 depicts the result of bonding a capping wafer to the substrateto enclose the microtube.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 10 represent steps in a process carried out to produce amicromachined tube (microtube) 50 (FIGS. 9 and 10) suitable for avariety of microfluidic devices. The drawings are drawn for purposes ofclarity when viewed in combination with the following description, andtherefore are not necessarily to scale.

FIG. 1 depicts limited portions of a pair of wafers 10 and 12 selectedfor processing in accordance with the invention. The wafers 10 and 12are both preferably silicon, though other materials can be usedincluding but not limited to Ge, SiC, GaAs, Si/Ge, diamond, sapphire,glass, ceramic materials, plastic materials, and titanium or othermetallic materials. In addition, the wafers 10 and 12 can be singlecrystal or polycrystalline. According to one embodiment of theinvention, the wafers 10 and 12 may be undoped, though in a preferredembodiment both wafers 10 and 12 are doped similar in type and level.The type (n or p-type) and doping level can be tailored as may berequired or desired by one skilled in the art. Suitable doping levelsfor the wafers 10 and 12 generally achieve resistivities of about 1 toabout 0.01 ohm-cm, with the exception of an etchstop region 14 overlyinga substrate region 16 of the wafer 12. The etchstop region 14 can be anoxide layer or a heavily doped epitaxial or diffused layer using knowndopants and doping techniques, such as p-type doping with boron,boron-germanium, etc. While shown as a surface region, the etchstopregion 14 could instead be formed as a buried layer of the wafer 12. Therole of the region 14 as an etchstop will become apparent from thefollowing discussion, though it will also become apparent that theregion 14 could be replaced by various other materials capable ofserving as an etchstop. To obtain a desired configuration and thickness,the wafers 10 and 12 can undergo various processes, including wetchemical etching (selective, timed, etc.), dry etching (e.g., ionmilling, plasma enhanced etching, reactive ion etching (RIE), deepreactive ion etching (DRIE), mechanical removal (grinding, polishing,etc.), chemical-mechanical polishing (CMP), etc. The thickness of thewafer 10 will subsequently limit the maximum height dimension of themicrotube 50 (as measured in a direction normal to the wafer surface) ofFIGS. 9 and 10. For microtubes of particular interest to the invention,the thickness of the wafer 10 is preferably in a range of about 100 toabout 1500 micrometers, though lesser and greater thicknesses are alsowithin the scope of this invention.

FIG. 2 shows the result of cleaning and then bonding the wafer 10 to theetchstop region 14 of the wafer 12 to form a wafer stack 18, resultingin the etchstop region 14 effectively becoming a buried layer within thewafer stack 18. Bonding can be accomplished by a variety of techniques,such as fusion, direct, anodic, solder, eutectic, and adhesive bonding.Silicon fusion bonding is the preferred method if the wafers 10 and 12are formed of silicon, as this technique can be performed at roomtemperature under vacuum or at ambient pressures with a plasma-assistbonding mechanism. As the intent of the bonding step is in part to burythe etchstop region 14, it is foreseeable that a wafer with a buriedetchstop region could be formed by various other processes that are alsowithin the scope of the invention. Furthermore, it will become apparentfrom the following discussion that the etchstop region 14 could beomitted.

Following bonding, a high temperature anneal/oxidation can be employedto strengthen the silicon fusion bond. FIG. 2 shows the wafer stack 18provided with masks 19 to protect its surfaces from attack during asubsequent etching step, the result of which is shown in FIG. 3. Themasks 19 can be oxide layers formed during the high temperatureanneal/oxidation step, though other processes known in the art can beused to mask the wafer stack 18 with a variety of masking materials,including but not limited to silicon nitride, combined silicon oxide andsilicon nitride, photoresists, polymers, metals, dielectrics, etc. FIG.3 shows the result of etching a channel 20 in a surface of the waferstack 18 formed by the wafer 10, for example, after removing part or allof the mask 19 overlying this surface. The channel 20 is represented ashaving an arcuate or curvilinear profile in cross-section. According toa preferred aspect of the invention, the etching technique used todefine the channel 20 is an isotropic process, preferably a dry etchingtechnique and more preferably a plasma etching technique using SF₆, CF₄,Cl₂, XeF₂, etc., though an isotropic wet etching technique could also beused. In each case, the isotropic etching process proceeds into thewafer 10 in all directions from the point at which etching is initiatedto achieve an arcuate or curvilinear profile shape, including but notlimited to the semicircular shape of the channel 20 shown in FIG. 3. Asshown in FIG. 3, the etching process is preferably terminated prior toencountering the etchstop region 14.

In FIG. 4, the etched wafer stack 18 of FIG. 3 is shown bonded to awafer 22 having a semicircular channel 28 defined in a substrate region24, and a mask 26 (for example, an oxide layer) on the surface of thesubstrate region 24 opposite the channel 28. The wafer 22 may be astandard silicon wafer, an epitaxial wafer, or processed as a waferstack similar to the wafer stack 18 of FIG. 3. Bonding of the waferstack 18 and wafer 22 can be accomplished by a variety of techniques,such as fusion, direct, anodic, solder, eutectic, and adhesive bonding.Silicon fusion bonding is again a preferred method if the wafer stack 18and wafer 22 are formed of silicon, and a high temperatureanneal/oxidation can be employed to strengthen the silicon fusion bond.The wafer 22 is preferably selected on the basis of having a channel 28of substantially equal width to the channel 20 of the wafer stack 18.The channels 20 and 28 can be matched via an alignment technique, andthe wafer stack 18 and wafer 22 bonded together to produce a waferstructure 30 within which the semicircular channels 20 and 28 define acircular passage 32 within the structure 30. It should be noted thatchannels 20 and 28 having cross-sectional profiles that deviate from asemicircular shape will yield passages 32 that deviate from a circularshape, for example, elliptical shapes. Furthermore, it is foreseeablethat the wafer stack 18 could be bonded to a flat surface of anotherwafer, yielding a semicircular passage. These and other cross-sectionalshapes incorporating a round profile are also within the scope of thisinvention.

FIG. 5 shows the result of thinning the wafer structure 30 by removingmaterial at both surfaces of the structure 30. The removal process atthe lower surface of the structure 30 (as viewed in FIG. 5) isrepresented as having been terminated at the etchstop region 14.Suitable etchants for this process will depend on the type of materialused to form the etchstop region 14, for example, an oxide layer or aheavily doped p-type silicon. Those skilled in the art will appreciatethat lapping, polishing, grinding, wet or dry etching, or a combinationof these techniques could be used to thin the wafer structure 30, withor without the presence of the etchstop 14. In the embodiment shown inFIG. 5, the thickness of the etchstop region 14 affects the minimumlower wall thickness of the tube 50 below the passage 32. Removal of thesubstrate region 24 opposite the etchstop region 14 can be by lapping,polishing, grinding, wet or dry etching, or a combination of thesetechniques, or through the presence of a buried etchstop (not shown)originally present in the wafer 22 similar to the wafer stack 18. Atimed etch or timed mechanical removal process can also be used toensure the remaining surface region 24 defines a suitably thick wallabove the passage 32. Suitable thicknesses for the tube wall will dependon the particular application for the microtube 50, with particularlysuitable thicknesses believed to be about ten to a few hundredmicrometers.

Following bonding of the wafer stack 18 and wafer 22 (FIG. 4) andoptionally thinning the resulting wafer structure 30, the passage 32 andthe surrounding structure can conceivably have a form suitable for usein a variety of microfluidic devices. According to a preferred aspect ofthe invention, FIGS. 6 through 10 depict further processing stepssuitable for further defining a microtube 50 and a microfluidic devicethat utilizes the microtube 50. FIG. 6 shows the result of masking andetching the surface of the wafer structure 30 opposite the etchstopregion 14 to establish what will become the lateral walls of themicrotube 50. For this step, a plasma etch process and a resist mask(not shown) may be employed, though other masking materials andtechniques could foreseeably be used, such as an oxide layer,combination of resist and oxide layer, etc. As evident from FIG. 6, theetching step is used to only partially etch through the thickness of thewafer structure 30. The depth of this etch is dependent on the thicknessand strength of the wafer structure 10 desired for subsequentfabrication and handling. In FIG. 6, less than half the thickness of thewafer structure 30 on either side of the passage 32 has been etched,leaving side portions 34 that interconnect the passage 32 to theremainder of the wafer structure 30.

FIGS. 7 and 8 represent a particular example in which the passage 32 hasbeen defined within the wafer structure 30 to have a U-shapedconfiguration (when viewed from above) comprising a pair of leg portions32A and an interconnecting distal portion 32B. As will be discussedbelow, other configurations are possible and within the scope of theinvention. FIGS. 7 and 8 represent cross-sections of the entire waferstructure 30 (instead of the partial sections represented in FIGS. 1through 6) that are taken transverse to each other, with FIG. 7 being across-sectional view transverse to and through the leg portions 32A ofthe U-shaped passage 32 and FIG. 8 being a cross-sectional view parallelto the leg portions 32A and transverse to and through the distal portion32B.

FIGS. 7 and 8 represent the wafer structure 30 after being flipped andbonded to a micromachined and metallized substrate 36, with the resultthat the portion of the wafer structure 30 containing the tube passage32 is cantilevered over a recess 44 in the substrate 36. The tube wallof the wafer structure 30 that faces the substrate 36 is represented ashaving an electrically conductive layer 40. As evident from FIG. 8, theconductive layer 40 provides an electrical path that connects themicrotube 50 and a metal contact 42 on the substrate 36, enablingelectrical grounding or biasing of the microtube 50. Various conductivematerials can be used as the conductive layer 40, which may or may notbe electrically insulated from the remainder of the wafer structure 30.Furthermore, the wafer structure 30 may be sufficiently doped (if formedof a semiconductor material) or otherwise formed of an electricallyconductive material to render the layer 40 unnecessary. The substrate 36may be formed of a variety of materials, including Pyrex, borofloat,quartz, or other glass-type wafer, silicon, SOI, plastic, ceramic, oranother material. According to a preferred aspect of the invention, thesubstrate 36 is a glass wafer. A variety of bonding techniques can beemployed for this purpose, with anodic bonding being preferred.According to an alternative aspect of the invention, the substrate 36 isa silicon wafer on which a dielectric coating or oxide layer has beenformed to provide an electrical insulating layer 38 for the metalcontact 42 as well as other metallization on the substrate 36 formingelectrical runners, bond pads, etc., for the microfluidic device. Avariety of bonding techniques can be employed for this purpose, withfusion bonding being preferred.

In the U-shaped configuration of the passage 32 represented in FIGS. 7and 8, a side portion 34A of the wafer structure 30 is surrounded onthree sides (in the plane of the wafer structure 30) by the leg anddistal portions 32A and 32B of the tube passage 32, and another sideportion 34B surrounds the tube passage 32 along its outer perimeter(again, in the plane of the wafer structure 30). By removing the sideportions 34A and B, the external shape of the microtube 50 will alsoapproximate a U-shape similar to that of the tube passage 32. Inlet andoutlet holes 46 (one of which is shown in FIG. 8) can be formed at thistime by etching, preferably DRIE.

FIG. 9 shows the result of masking and etching the remainder of thewafer structure 30 to remove the side portions 34 and complete themicrotube 50 and its outer periphery, including the external surfaces ofthe walls that surround the tube passage 32. For this process, a mask(not shown) can be aligned through the substrate 36 to the edges of theside portions 34 or to metallization or the recess 44 on the surface ofthe substrate 36 using double-side alignment tools or another similartechnique known in the art. Alternatively, IR alignment can be employed.After alignment and development, the side portions 34 of the waferstructure 30 can be removed, preferably by DRIE plasma etching. As analternative method, a single plasma etch could be employed before orafter bonding of the wafer structure 30 to the substrate 36, and tabs orthick scribe street rims could be employed to mechanically reinforce thewafer structure 30 after etching prior to bonding.

With the microtube 50 cantilevered over the recess 44 in the substrate36 as represented in FIG. 9, the microtube 50 can be vibrated and itsmovement induced relative to the substrate 36 in a directionperpendicular to the plane of the microtube 50. For this purpose, FIGS.7, 8 and 9 show drive and sensor electrodes 52 and 54 formed within therecess 44 for electrostatic coupling with the microtube 50. Theconductive layer 40 on the lower surface of the microtube 50 facing theelectrodes 52 and 54 can serve as an electrode on the microtube 50 forelectrostatically driving the freestanding portion of the microtube 50with the drive electrode 52. Alternatively, the microtube 50 could beformed to be electrically conductive, such as doped silicon, to enableelectrostatic driving of the microtube 50 without a separate electrode.It should be noted that vibration or other desired movement of themicrotube 50 relative to the substrate 36 can be induced in the tube 50by means other than electrostatically, including but not limited topiezoelectrically, piezoresistively, acoustically, ultrasonically,magnetically, optically, or another actuation technique. Movement of thetube 50 can be sensed capacitively, piezoelectrically, piezoresistively,acoustically, ultrasonically, magnetically, optically, or anothersensing technique. These actuation and sensing techniques can be appliedto the microtube 50 in combination with the substrate 36, or without thepresence of the substrate 36, or in combination with another substratesuch as a product package.

In addition to the U-shape represented in the Figures, the microtube 50and its passage 32 can have a variety of other shapes (in plan view),including but not limited to the C-shaped tubes of U.S. patentapplication Ser. Nos. 11/620,908, 12/267,263 and 12/369,118, doubletubes of U.S. patent application Ser. Nos. 12/143,942 and 12/267,263,S-shaped tubes of U.S. patent application Ser. Nos.11/620,411 and12/267,263, and straight tubes of U.S. patent application Ser. No.12/369,510. The contents of these applications relating to theconfigurations and use of their microtubes are incorporated herein byreference. Notably, such configurations for the microtube 50 do notnecessarily require the presence of the substrate 36. For example, usingstraight tubes of the type disclosed in U.S. patent application Ser. No.12/369,510, the microtube 50 can be vibrated and its vibration sensedwithin the plane containing the microtube 50.

Finally, FIG. 10 shows the result of bonding a capping wafer 56 to thesubstrate 36 to enclose the microtube 50, preferably vacuum sealing themicrotube 50 between the substrate 36 and capping wafer 56 in order toenhance the dynamic performance of the microtube 50 if the microtube 50is desired to vibrate, for example, in accordance with U.S. Pat. Nos.6,477,901, 6,647,778, 7,351,603 and 7,381,628. A variety of materialscan be considered for the capping wafer 56, including but not limited tosilicon, glass, ceramic, and plastic wafers that can be processed tohave a sufficiently deep cavity sufficient to accommodate the microtube50. The capping wafer 56 is shown as having an integrated getter 58 toimprove vacuum quality in accordance with known practices. Depending onthe materials of the substrate 36 and capping wafer 56, sealing of thecapping wafer 56 to the substrate 36 can be by glass frit sealing,eutectic bonding, solder bonding, anodic bonding, or other bondingtechnique known in the art. Alternatively, this step can be omitted ifan acceptable vacuum can be formed without wafer-to-wafer bonding. Inaddition, the capping wafer 56 can be omitted and enclosure of themicrotube 50 can be performed in a subsequent packaging step, such asbut not limited to IC packaging (e.g., an IC package with a Kovar lid)or product packaging.

While the invention has been described in terms of a particularembodiment, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

The invention claimed is:
 1. A microtube of a microfluidic device, themicrotube comprising: a substrate structure comprising first and secondsubstrates bonded together at respective first surfaces thereof, each ofthe first and second substrates having a corresponding second surfaceoppositely disposed from the first surface thereof so as to define,respectively, first and second outer surfaces of the substratestructure; a passage within a first region of the substrate structure,the passage having a cross-sectional profile defined by first and secondchannels in, respectively, the first surfaces of the first and secondsubstrates, each of the channels having an arcuate cross-sectionalprofile and being enclosed between the first and second substrates todefine the passage; and side portions within a second region of thesubstrate structure, the side portions being separated by the passageand by the first region of the substrate structure within which thepassage is disposed, each of the side portions having an etched surfaceat the second outer surface of the substrate structure, the secondregion of the substrate structure defined by the side portions having athickness that is less than a thickness of the first region of thesubstrate structure within which the passage is disposed.
 2. Themicrotube according to claim 1, wherein the passage is present in aportion of the first region of the substrate structure that protrudesbeyond the etched surfaces of the side portions.
 3. The microtubeaccording to claim 1, wherein the second outer surface of the substratestructure has a stepped profile as a result of the thickness of thesecond region of the substrate structure being less than the thicknessof the first region of the substrate structure.
 4. The microtubeaccording to claim 1, wherein the arcuate cross-sectional profiles ofthe channels are substantially identical.
 5. The microtube according toclaim 4, wherein the passage has a circular cross-sectional shape. 6.The microtube according to claim 4, wherein the passage has anelliptical cross-sectional shape.
 7. The microtube according to claim 1,wherein the first and second substrates are formed of materials chosenfrom the group consisting of silicon, Ge, SiC, GaAs, Si/Ge, diamond,sapphire, glass, ceramic materials, plastic materials, and metallicmaterials.
 8. The microtube according to claim 1, wherein the firstsubstrate comprises a buried layer.
 9. The microtube according to claim1, further comprising a third substrate bonded to the substratestructure such that the first region of the substrate structurecontaining the passage is spaced apart from a surface of the thirdsubstrate and a portion of the microtube projects over the surface ofthe third substrate so as to be capable of movement relative thereto.10. The microtube according to claim 9, wherein the third substratecomprises at least one of a semiconductor, silicon, glass, ceramic, orplastic material.
 11. The microtube according to claim 9, wherein thethird substrate comprises a semiconductor material and an insulatinglayer to which the substrate structure is bonded.
 12. The microtubeaccording to claim 9, further comprising means on the substratestructure and on the third substrate for electrically connecting themicrotube to metallization on the third substrate.
 13. The microtubeaccording to claim 9, further comprising a capping wafer bonded to thethird substrate and enclosing the microtube.
 14. The microtube accordingto claim 13, wherein the microtube is vacuum sealed within a cavitydefined by and between the third substrate and the capping wafer. 15.The microtube according to claim 9, further comprising holes in thethird substrate that fluidically interconnect opposite ends of thepassage of the microtube and define fluid inlet and outlet ports of themicrotube.
 16. The microtube according to claim 1, further comprisingmeans for vibrating the microtube perpendicular to a plane containingthe microtube.
 17. The microtube according to claim 1, furthercomprising means for vibrating the microtube within a plane containingthe microtube.